1. Field of the Invention
This invention relates to a method and apparatus for producing ceramic-based electronic and other components, and more particularly to a particular combination of ceramic compositions useful for fixtures for heat treating such components.
2. Description of Related Art
Components in electronic circuitry utilize ceramic materials for both substrates and semiconductor packages. However, the most common type of ceramic-based electronic components for which the present invention is useful in heat treating are ceramic capacitors, resistors, thermistors or the like which are typically made up of multiple layers of oxide or nonoxide ceramics having suitable dielectric properties. Common sizes are 0.08.times.0.05 in., 0.125.times.0.063 in. and 0.5.times.0.225 in. Multilayer ceramics are also used as substrates for integrated circuit packages. Alumina-based ceramics are widely employed, as well as mullite (3Al.sub.2 O.sub.3.2SiO.sub.2), beryllia (BeO), aluminum nitride (AlN) and various other well known glass-ceramic materials, depending on the dielectric constant, coefficient of thermal expansion, and other properties desired. Different classifications for ceramic dielectric materials are Class I dielectrics with low k (dielectric constant) values made by mixing magnesium titanate with calcium titanate; Class II dielectrics with high k values (also known as ferroelectrics) based on barium titanate, optionally with additions of barium stanate, barium zirconate or magnesium titanate; and Class III dielectrics. Other ceramic materials used in electronic applications include magnesia; nickel manganates (NiO.MnO.sub.2); zirconia (ZrO.sub.2); yttria (Y.sub.2 O.sub.3); lead compounds such as lead oxide (PbO), lead non-oxides, lead zirconium titanates, lead titanates, and lead metaniobates; beryllium compounds such as BeO; ferrous oxide compounds (e.g., magnetic and non-magnetic FeO compounds, including those in mixture or compound with Zn and/or Mn) and ferrites. Ferrites are a well known class of ceramics having the spinel cubic structure of the general formula XFe.sub.2 O.sub.4, where X may represent Ba, Zn, CD, Cu, Mg, Co, Ni, Mn, Fe or a mixture of these or other ions.
In processing such ceramic-based electronic components, the parts in the "green" state are fired one or more times to temperatures of approximately 1000.degree. C.-1700.degree. C. and higher, more typically 1100.degree. C.-1500.degree. C., to achieve vitrification, sintering and/or densification. Total cycle times for heat treatment are typically eight (8) hours or longer, although shorter times may be used where there is only a small mass of product. Firing may be done under a vacuum or protective atmosphere but is typically done in air. Standard type furnaces or kilns employed in the industry are typically either the pusher or tunnel type and the periodic or batch type.
The devices or fixtures by which the ceramic components are physically supported during the firing process are generally termed furnace or kiln furniture. Other well known nomenclature is utilized for various configurations and types of fixtures such as saggers, setters, and plates, and varieties of substrates with rails or sidewalls. Processes used for manufacturing prior art monolithic furnace fixtures include press cast molding, powder roll compaction, tape casting, slip casting or extrusion of the green ceramics to the desired shape, followed by firing of these materials in the range of temperatures given previously. A wide variety of ceramic materials have been employed such as the aforementioned and other alumina and alumina-based ceramics, as well as zirconia and magnesia. In some instances, powdered forms of these ceramics have been applied to furnace fixtures either as dry powders or aqueous washes which are then dried to leave a powder residue, to prevent sticking of the components to the fixture surface.
Although the prior art methods of using these monolithic furnace fixtures have not changed dramatically over the years, there has been a long sought need to reduce furnace time and associated energy costs to maximize productivity in processing ceramic-based electronic components. With some types of fixture materials, especially those which have low reactivity with typical ceramics used in electronics, this has been difficult because of the relatively thick fixture cross sections necessitated for purposes of maintaining strength and thermal shock resistance. In some instances, the fixture panels had high porosity and a rough surface, and were not considered mechanically suitable for use in thin, large panels which are desirable for maximizing the number of components which may be placed in the furnace. The result has been that a relatively large amount of furnace heat goes to heating the fixtures themselves, which not only costs more fuel but also penalizes the process by requiring longer heat up and cool down time for the combined mass of fixtures and electronic components. Ceramic materials which have greater strength, can be easily processed to flatness and other dimensional parameters, and can withstand cyclic exposure to high temperature firing such as alumina do not have the degree of chemical inertness needed for processing many of the variety of ceramics used in electronics, and therefore have only limited potential for use.
Similar problems have been encountered in firing other ceramic components from the green state. Other components include those made from zirconium oxide (ZrO2), such as oxygen sensors used to regulate internal combustion engines.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a process for heat treating ceramic-based electronic and other components which reduces the time and energy required to fire the components, while at the same time providing a surface for contacting the component which is relatively inert so as to avoid chemical contamination of the ceramics employed in the components.
It is another object of the present invention to provide furnace fixtures for processing ceramic-based electronic and other components which have a relatively thin cross section while having a high degree of strength and resistance to thermal cycles at the temperatures and conditions employed in heat treating such components.
It is a further object of the present invention to provide furnace fixtures for processing ceramic-based electronic and other components which may be produced to close tolerances for flatness and other dimensional parameters.
It is yet another object of the present invention to provide a process for heat treating ceramic-based electronic and other components in which the furnace fixtures eliminate sticking of the components to the fixture surface and have a long working life.
It is a further object of the present invention to provide furnace fixtures for processing ceramic-based electronic and other components in which the total volume of fixture material is reduced and the volume of furnace space which is available for components to be heat treated is increased.